Optimizing rotational grazing in sheep management systems

Abstract

Rotational grazing on sheep farms was modeled using an iconic simulation modeling package. An event-driven modeling approach allowed the simulation of individual paddocks on a farm and their grazing. It permitted the effects of different levels of herbage allowance to be tested and measured with respect to pre- and post-grazing herbage mass, rotation length, diet quality, animal performance, and the enterprise gross margin. Validation of the model against three experimental data sets indicated a good fit. A genetic algorithm was implemented to enable the optimum sequence of control and state variables to be determined for the grazing system. This was used to calculate the optimum stocking rate and monthly sequence of herbage allowance, nitrogen application, winter supplementation, and lamb drafting weight for 12 months on a theoretical farmlet. The farmlet was stocked with 17.5 sheep (ewes)/ha at an initial liveweight of 55 kg, a starting pasture dry matter cover of 1500 kg/ha, and 130% lambing. Pasture allowances suggested by the optimization were consistent with the recommendations provided to sheep farmers, while nitrogen applications were higher than expected, reflecting the prevailing high lamb prices compared to the cost of nitrogen boosted pasture.